Here’s the most recent issue (11:43am) of the forecasts for Tuesday and Wednesday that we’re providing to those involved in the emergency response. It has as much detail as we can sensibly provide in a worded forecast about the winds and potential for dust.

Wednesday – 02 March 2011
High cloud. Brief rain around dawn, then skies clearing and becoming fine late morning.
Northerly 45 km/h gusts 65 km/h turning northwest about dawn. Westerly may rise to 55 km/h gusts 80 km/h at times from late morning to early afternoon. Winds easing to westerly 30 km/h late afternoon and winds easing to 10 km/h in the evening. These strong winds during the day may raise clouds of swirling dust.
Note that winds will be stronger about the higher parts of the Port Hills with northwest gales gusting 110 km/h during the morning.
Rainfall amount: 2-5mm
Maximum temperature 27C
Minimum temperature overnight 9C

As of the time of publishing this blog (3:00pm Monday 28 February 2011), there’s a Severe Weather Warning in effect for severe gales in inland areas of Canterbury and the higher parts of Banks Peninsula, but not for Christchurch. The Warning has been issued well in advance of the severe gales: they are expected on Tuesday night and Wednesday morning.

In particular, the Warning states “Severe gales are not expected in Christchurch city but west to northwest wind gusts could reach about 90km/hr from early Wednesday morning until early Wednesday afternoon.” We’re doing our best to nail down the timing: make sure you keep up-to-date with the latest Severe Weather Warning.

Right now, there’s not much wind in Christchurch, and MetService isn’t expecting it to pick up significantly until later on Tuesday.

The threshhold for blowing dust is around 30km/h, though a lot of other factors influence how much dust gets into the air. So, it is reasonable to expect dust clouds around from later on Tuesday (when the wind picks up) to later on Wednesday (when it eases off again).

Wind gusts are the other thing to be mindful of, particularly since there’s quite a few tents up around the Christchurch area. Now’s the time to make sure your guy ropes are well secured.

If you’re interested in why MetService expects Christchurch to be less windy during this event than some other parts of Canterbury, here’s the explanation.

The Christchurch area is expected to experience steady northerlies, around the 40km/h mark, from about sunset on Tuesday 01 March to about sunrise on the morning of Wednesday 02 March. Near-surface effects (in particular, the likely strengthening of an inversion during the night hours) are likely to keep the stronger winds further up in the atmosphere from getting down to the land surface. During Wednesday morning, there’s a general shift in the Canterbury area to winds which are from just north of west, so that they travel across the plains (instead of over the sea) before reaching Christchurch. This, together with the weakening of the inversion through daytime heating (the sun warms the ground, which in turn warms the air above it), is likely to allow the stronger winds from higher up to “mix down” to the surface more easily. This “mixing down” is a turbulent process: that is, it happens from time to time and from place to place, not constantly. In this particular event, the gusts experienced in Christchurch will be little bursts of stronger wind mixed down to the surface.

This is the last post in this series of forecasts for Christchurch. The forecast team are paying extra special attention to the Christchurch forecast and today we’ve added high resolution radar imagery to metservice.com. We’re also putting considerable effort into providing forecast support to those involved in the emergency response.

Mercifully, there has been no severe weather in the Christchurch area since Tuesday’s earthquake. The Severe Weather Outlook and the Severe Thunderstorm Outlook show that none is currently on the horizon. Both of these outlooks are issued at least once a day by members of MetService’s Severe Weather Team; text versions are available by email.

Summary: Friday to Tuesday
Dry weather is expected until the end of Monday; rain is likely on Tuesday. No strong winds expected. Cool some nights.

Summary: Friday to Tuesday
Rain easing to showers late this afternoon, then clearing. After that, dry weather is expected until the end of Monday; rain is possible on Tuesday. No strong winds expected. Cool most nights.

Friday
Rain, with briefly moderate falls around the middle of the day, easing to showers late afternoon. Southwesterlies, fresh for a time around the middle of the day. This evening, showers clearing (but remaining cloudy) and southwerlies dying away.

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